There have recently been seen some concrete or steel structures of which properties at the time of designing are no longer maintained due to deterioration of structural members over time. Reinforcement or repair of such structures has been effected, such as reinforcement for improving their earthquake-proof properties, repair for suppressing deterioration of structural members, or reinforcement for improving their functionality.
Conventional reinforcing methods include, for example, affixing sheets containing reinforcing fibers, such as reinforcing fiber sheets and/or fiber-reinforced plastic plates, over the surface of a structure to be reinforced to integrate the sheet with the structure. This reinforcing method has been generally adopted with many successful results. On the other hand, methods for reinforcing a concrete surface of tunnel inner walls are also known, which include lining the outer concrete surface with shotcrete or PC plates, and optionally arch-setting with liner plates and H-steels, and splicing steel plates. Also known are methods for repairing or reinforcing headrace tunnels and the like for resolving problems such as water leakage, decline in strength due to internal and external pressures, or decrease in actual water delivery. These methods include, for example, a spraying method wherein steel fiber-containing mortar or steel fiber-containing concrete is sprayed over the surface of the existing concrete lining, a painting method wherein resin mortar or steel fiber-containing mortar is painted over the surface, a casting method, and an affixing method.
Among the various methods mentioned above, for example, when the method of affixing sheets containing reinforcing fibers over a structure surface is performed using sheets that are hard to break and high in tensile strength, the sheets provide excellent reinforcing effect, as long as the sheets are fixed to the structure. In the final stage of the structure life, however, the sheets containing reinforcing fibers tend to separate from the structure before they are broken, loosing the reinforcing effect, which ultimately leads to breakdown of the structure.
For the purpose of overcoming such drawbacks, there have been proposed methods for preventing separation of the sheets containing reinforcing fibers from the structure. Such methods include, for example, providing an additional material containing reinforcing fibers for fixing the sheets for reinforcement to the structure, or fixing the sheets to the structure by means of anchors or metal plates. With these methods, however, maximum use of the sheet strength is hard to be enjoyed, or the working process maybe complicated. Further, in the case of tunnel structures having a curved surface, even a small displacement will tend to cause separation of the sheets from the inner wall of the tunnel structure, and thus the reinforcing effect is hard to obtain.
There is also known a method wherein the sheets containing reinforcing fibers are stuck to the inner wall surface of a structure with an adhesive. However, conventional adhesives are only for sticking the sheets, and little cushioning effect is provided between the sheets and the structure. Thus sufficient and long-lasting prevention of separation is not achieved. Incidentally, conventional adhesives can only form a layer having a tensile elongation of less than 5% at maximum load at 23° C.